Method and composition for electroplating cadmium (b)

ABSTRACT

AN AQUEOUS BATH FOR PRODUCING BRIGHT, ADHERENT CADMIUM ELECTRODEPOSITS UPON METALLIC SURFACES CONTAINS CADMIUM ION, CHLORIDE ION, A NICOTINIC ACID AND AN AMINOPOLYACETIC ACID TYPE OF CHELATING AGENT. THE BATH IS HIGHLY CONDUCTIVE AND OPERABLE AT LOW VOLTAGES, AND HAS A PH WHICH IS ACIDIC OR SLIGHTLY ALKALINE.

United States Patent Office 3,577,327 IVIETHOD AND COMPOSITION FOR ELECTROPLATIN G CADMIUM (B) Tibor Joachim, Vernon, and Merton M. Beckwith, Tolland, Conn., assignors to Conversion Chemical Corporation, Rockville, Conn.

No Drawing. Filed Feb. 19, 1969, Ser. No. 800,725 Int. Cl. C23b 5/10, 5/12, 5/46 US. Cl. 204-50 15 Claims ABSTRACT OF THE DISCLOSURE An aqueous bath for producing bright, adherent cadmium electrodeposits upon metallic surfaces contains cadmium ion, chloride ion, a nicotinic acid and an aminopolyacetic acid type of chelating agent. The bath is highly conductive and operable at low voltages, and has a pH which is acidic or slightly alkaline.

BACKGROUND OF THE INVENTION A variety of baths have been widely employed for electroplating cadmium onto metallic substrates, and most commonly sulfates and cyanides are utilized as the primary electrolytes therein. Generally, the cyanide baths have proven quite effective and satisfactory despite certain significant objectionable features, such as high toxicity, difiiculty of disposal, low current efiiciency and hydrogen embrittlement of certain steels. The sulfate baths overcome many of the objectionable features of the cyanide baths, but normally they exhibit relatively low throwing power, poor efficiency at low current densities and secondary anode corrosion which results in an increase of metal in the bath.

Accordingly, it is an object of the present invention to provide a novel chloride plating bath for producing adherent cadmium deposits on various metals.

It is also an object of the invention to provide such a bath for producing bright deposits of cadmium which are ductile.

Another object is to provide a cadmium plating bath which is operable with high current efficiency over a wide range of current density, which affords excellent throwing power and operability at low voltages.

Another object is to provide such a bath which is effective in depositing cadmium upon cast and malleable irons and which reduces hydrogen embrittlement to a level below that encountered 'with conventional cyanide baths in electroplating certain ferrous alloys.

A further object is to provide a method for electroplating cadmium onto metallic workpieces utilizing such a chloride plating bath so as to obtain highly advantageous bright, adherent and ductile cadmium deposits at relatively low cost and over a broad range of current densities.

SUMMARY OF THE INVENTION It has now been found that the foregoing and related objects can be readily attained in an aqueous bath comprising about 7.5 to 60.0 grams per liter of cadmium ion, about 16.0 to 135.0 grams per liter of chloride ion, at least about 1.0 gram per liter of a nicotinic acid and about 0.16 to 1.2 gram moles of an amino-polyacetic acid type of chelating agent. The bath is operable at a pH of about 5.0 to 7.5 and at current densities ranging, as a practical matter, from about 5.0 to 70.0 amperes per square foot.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The components which are essential for the proper operation of the baths of the present invention are cadmium and chloride ions, a nicotinic acid and an amino- 3,577,327 Patented May 4, 1971 polyacetic acid-type of chelating agent. Ammonium ion has also been found to be beneficial, and as a result baths which avoid the presence of unnecessary and possibly interfering ions may desirably be formed by the admixture of cadmium chloride, ammonium chloride, the nicotinic acid and the chelating agent. However, cadmium oxide may furnish the cadmium ion vvithout detriment, and its use may be preferred from an economic standpoint. Other ammonium compounds may supplement ammonium chloride and, for example, ammonium hydroxide may be added when adjustment of the pH to a higher value is necessary. In the event that the bath is more basic than desired, hydrochloric acid is a suitable reagent for lowering the pH.

As has been pointed out, the baths may contain about 7.5 to 60.0 grams per liter of cadmium ion, and preferably the concentration of the cadmium ion is about 15.0 to 40.0 grams per liter.

A cadmium ion concentration higher than about 60.0 grams per liter tends to produce a deposit which is course, grainy and spongy, whereas the current densities attainable in baths containing less than about 7.5 grams per liter thereof tend to be too low and result in non-uniform coatings which exhibit evidence of burning in some areas.

Although the chloride ion concentration may range from about 16.0 to 135.0 grams per liter, the baths preferably contain about 33.0 to 105.0 and, most desirably, about 50.0 to 66.0 grams per liter thereof. It has been found that use of less than about 16.0 grams per liter of this ion produces a poorly conductive bath which is operative only over a narrow range of current density, and which produces coatings of marginal quality. Raising the concentration thereof beyond about 135.0 grams per liter produces little, if any, additional increase in current density, and at the same time tends to cause a very undesirable sludge to build up and coat the anode surface. The presence of ammonium ion has a very desirable effect upon the operation of the bath and upon the quality of the deposit, so that ammonium chloride is preferred as either the sole source of chloride ion or to increase the concentration thereof if another source of chloride ion is present, such as cadmium chloride. Too much of the ammonium ion tends to cause precipitation in the bath and is to be avoided; thus, the ammonium ion is advantageously present in an amount of about 0.5 to 4.0, and preferably about 1.0 to 3.0 gram moles per liter.

The baths must contain at least 1.0 gram per liter of a nicotinic acid if they are to be effective in providing the objects of the invention. However, as the concentration is decreased to 1.0 gram per liter, the conditions at which the bath is operative become quite limited; accordingly, it is preferred to use about 2.0 to 20.0 grams per liter and most desirably about 4.0 to 12.0 grams per liter. Although more of this component can be employed, there is little advantage in doing so and there is a tendency for cathode efliciency to be sacrificed. Moreover, too much of this component causes a great deal of internal stress to be created in the deposit, which tends to cause poor adherence and peeling of the deposit from the metal. Therefore, 20.0 grams per liter constitutes a practical and economical upper limit.

It should be understood that the term a nicotinic acid as employed herein includes not only nicotinic acid per se but also isonicotinic acid, derivatives and mixtures thereof. In fact, if only one of the acids is used, isonicotinic is the preferred compound. However, the best results are attained if a mixture of the two acids is utilized, and they are most advantageously used in a weight ratio of nicotinic acid to isonicotinic acid of about 2.0 to 4.0: 1.0; ideally, the ratio thereof is about 30:10.

Exemplary of the aminopolyacetic acid-type of chelating agents which are suitable for use herein are nitrilotriacetic acid, ethylenediaminetetraacetic acid, trisodium-N- hydroxyethyl ethylenediaminetriacetate, and mixtures thereof, and it should be noted that the term aminopolyacetic acid-type of chelating agents encompasses not only the acids but also water-soluble salts thereof, such as the acetates. If only one species of this type of chelating agent is included in the bath, the preferred compound is nitrilotriacetic acid; however, it is even more desirable from the standpoint of achieving maximum brightness in the deposits, particularly at low current densities, to utilize a mixture of nitrilotriacetic acid with one or more of the other aminopolyacetic acid-type of chelating agents. Thus, although it may be possible to achieve a comparable level of brightness with nitrilotriacetic alone at low current densities, the cost of the necessary amount of nitrilotriacetic acid tends to be prohibitive and the efficiency of the bath at the higher current densities is much lower than when a second chelating agent is used (i.e., it drops from above 90 percent to about 3040 percent under the same conditions of current density, temperature, etc.).

Whichever chelating agent or combination is employed, the amount thereof will generally range from about 0.16 to 1.2 gram moles per liter which, in the case of nitrilotriacetic acid, is about 30.0 to 210.0 grams per liter. The preferred concentration of the chelating agent is about 0.5 to 1.0 molar, and most desirably it is about 0.75 to 0.85 molar. If a second aminopolyacetic acid-type of chelating agent is used in combination with nitrilotriacetic acid, it may suitably be present in a concentration of up to about 0.14 molar, but preferably the concentration thereof will be about 0.02 to 0.12 mole per liter.

The concentration of the chelating agent is most appropriately related directly to the concentration of the cadmium ion. Satisfactory operation of the bath and acceptable deposits of cadmium depend upon the adequate chelation of that ion, so that at least 1.6 gram moles of chelating agent must be present per gram mole of cadmium; a lower ratio tends to produce deposits which are spongy, coarse, and porous, particularly at the extremes of current density. On the other hand, if the amount of chelating agent is too large, the plating of,

cadmium is inhibited to an unsatisfactory extent and there is a strong tendency for gas evolution to occur at the cathode. Generally, the molar ratio of a single chelating agent to cadmium ion should be between about 1.6 and 2.8:l.0; preferably, the ratio is about 2.0 to 2.6:1.0, and most desirably it is about 2.2 to 2.4: 1.0. At a ratio above about 2.811, satisfactory deposits are not produced at the low end of the current density range; furthermore, efficiency begins to fall off at a ratio above about 26:10. As has been mentioned, the deposits produced when a ratio of less than about 1.6: 1.0 is used are not satisfactory; and below about 20:10, they begin to get fairly dull. Even if nitrilotriacetic acid is used with a second chelating agent it should nevertheless be provided in the indicated ratios, with the ratio of the second agent to cadmium ion being about 0.12 to 1.0:1.0; preferably that ratio will be about 0.25 to 0.5:1.0, and most desirably about 0.30 to 0.40: 1.0. In a ratio above about 1.0: 1.0 misplating tends to occur, and the efficiency is undesirably low even within the fairly narrow range of high current density at which plating is feasible.

Although it is not absolutely essential, the best deposits in terms of maximum specular brightness are obtained if a quantity of a suitable polyhydric acid is also included. Exemplary polyhydric acids are citric acid, boric acid, malic acid, tartaric acid and mixtures thereof; the most outstanding results have been found to result from the use of citric acid. Normally such an acid will be included in the bath in a concentration of about 6.0 to 80.0 grams per liter, but preferably its concentration will be about 15.0 to 60.0 grams per liter. Although the theory underlying the effectiveness of these polyhydroxy acids is not fully understood, it is believed that they may function as buffering agents and/ or that they have a weak chelating effect and tend to maintain solubility at the anode.

In addition to the foregoing components, the baths of the present invention may contain conventional surfactants, other conventional brighteners, buffers and various additional additives which are commonly included in baths of this type. It has, in fact, been found to be particularly desirable to include a small amount of an anionic wetting agent, which effectively counteracts any tendency of the baths to cause pitting of the plated surface. More specifically, the anionic alcohol sulfates sold by E. I. du Pont de Nemours & Co. under the Duponol trademark are quite effective. For example, about 2.0 to 5.0 milliliters per liter of a 2.5 percent solution of Duponol Sn(sodium salt of mixed long-chain alcohol sulfates) or of a 10.0 percent solution of Duponol (sodium salt of octyl alcohol sulfate) provide significant benefits. However, care must be exercised, because too much of such a component can result in misplating or in a bath with very poor current efficiency.

Since the basic components of these baths are normally solid materials, the solutions may conveniently be prepared from dry formulations containing the proportions of the components necessary to produce the desired concentrations upon dissolution in water. A suitable dry formulation might, for example, contain cadmium oxide, ammonium chloride, nitrilotriacetic acid, ethylenediaminetetraacetic acid and citric acid. The proper amount of the mixed powder would then be added to water and the pH of the solution adjusted if necessary. Although they are also dry materials, the nicotinic and isonicotinic acids are most appropriately introduced as ammoniacal solution into the separately prepared solution of the other components.

A specific formulation which has proven useful is as follows:

Cadmium oxide-40.0 g./l.

Ammonium chloride80.0 g./l. Nitrilotriacetic acid-157.0 g./l. Ethylenediaminetetraacetic acid-32.0 g./l. Citric acid30.0 g./l.

Isonicotinic acid-10 g./l.

Nicotinic acid3.0 g./l.

Duponol-80 -3.O ml./ 1. (10% solution) Duponol-SN -3.0 ml./ 1. (2.5% solution) Water to one liter.

1 (E, I. du Pont de Nemours & C0. trademarks for anionic sulfate surfactants.)

For most effective operation, the temperature of the 'bath should be maintained at about 60 to 140 Fahrenheit, and the pH should be about 5.0 to 7.5. For optimum results a temperature range of about 80 to 120 Fahrenheit and a pH of about 6.0 to 6.8 are preferred, and most desirably the temperature will be about to Fahrenheit. If the concentrations of the various constituents indicated hereinbefore are maintained with the ranges specified, the baths are operable with high plating efficiencies over a broad range of current densities; i.e., from about 1.0 to 100.0 amperes per square foot. However, as a practical matter, the voltage is preferably adjusted to provide a current density of about 5 .0 to 70.0 amperes per square foot, and most desirably the baths are operated with current densities of about 25.0 to 40.0 amperes per square foot. The baths of the present invention are very stable; they exhibit excellent throwing power over a broad range of current density and allow efficient plating under a wide variety of conditions.

Although agitation is not necessary, it helps to produce high quality, uniform deposits and to avoid the development of anode sludge and film. Not only is agitation of the bath itself desirable, therefore, but the anode may also be agitated to reduce any tendency for sludge to accumulate thereat. Furthermore, agitation of the cathode (workpiece) may be desirable to obtain a uniform plate and to enable extension of the range of satisfactory current densities for a given bath to the higher levels of the indicated operable range.

Filtration is not essential but is normally beneficial, particularly when contamination of the bath is encountered frorn airborne impurities and by carry-over from other operations. For this purpose, various filter media including fabrics, porous stoneware and other conventional materials may be utilized. The need for correction for the depletion of the various components of the bath is best determined by a periodic quantitative analysis for the several components based upon the size of the bath and the plating rate.

Pure cadmium anodes should be employed with these baths to avoid introduction of impurities and degradation of the quality of the plate or of the components of the bath. Any metallic substrate that can be plated with cadmium using prior art baths may be coated in accordance with the present invention, and, as has been pointed out hereinbefore, these baths are particularly beneficially used for plating steel alloys which are susceptible to hydrogen embrittlement.

Illustrative of the efiicacy of the present invention are the following specific examples wherein all parts are parts 'by weight unless otherwise indicated.

EXAMPLE ONE An aqueous bath is prepared containing about 40 grams per liter of cadmium oxide, about 80 grams per liter of ammonium chloride, about 140 grams per liter of nitrilotriacetic acid and about one gram per liter of isonicotinic acid. The pH of the bath is adjusted to about 6.0 with ammonium hydroxide, and it is maintained at a temperature of about 80 Fahrenheit. A cast iron bar is immersed in the bath along with a pure cadmium anode, and a voltage i applied across the anode and the bar to effect plating for about 15 minutes at a current density of about 30 amperes per square foot. The bar is removed from the bath and is found to have a bright deposit of cadmium of about 0.4 mil thickness on its surface, which is adherent and relatively free of surface imperfections.

EXAMPLE TWO A bath is prepared in the same manner as is described in Example One, but 157 grams per liter of nitrilotriacetic acid are employed rather than 140 grams per liter. The bath additionally contains about 3 grams per liter of nicotinic acid, about 32 grams per liter of ethylenediaminetetraacetic acid, about 30 grams per liter of citric acid, about 3 milliliters per liter of a 10 percent solution of a sodium salt of octyl alcohol sulfate (Duponol 80 sold by E. I. du Pont de Nemours & Co.) and about 3 milliliters of a 2.5 percent solution of a sodium salt of mixed long-chain alcohol sulfate (Duponol SN sold by E. I. du Pont de Nemours & Co.) The bath is operated at about 100 Fahrenheit and at a pH of about 6.5.

A heat-treated tempered steel buckle is subjected to an alkali electrocleaning, rinsed in water, treated in an aqueous 5 percent citric acid dip, rinsed in water again, and plated in the bath using mild agitation and a pure cadmium anode. After plating for about 15 minutes at a current density of about 25 amperes per square foot to produce a deposit about 0.5 mil thick, the object is removed from the bath. The deposit is uniform and very bright; it is essentially free from pitting and other surface defects, and there is no evidence of burning. Upon testing, the deposit is found to be very adherent, and the level of hydrogen em'brittlement of the buckle is significantly lower than that encountered under comparable circumstances using a cadmium cyanide bath.

EXAMPLE THREE A bath containing the same components as that of Example Two is prepared in the same manner, but the concentration of components therein is reduced to half.

A steel panel is plated for about 6 minutes in a Hull Cell, the temperature of the bath being about Fahrenheit and the current being about 2 amperes. At the completion of the plating operation, the panel is removed from the cell and the deposit of cadmium is found to be bright and uniform over the entire range of current density. The panel is bent through an angle of lengthwise (i.e., to bisect the panel through the Zones of current densities), and no peeling of the coating is detected. Thereafter, the panel is hammered along the fold to the point at which complete fracture occurs. No peeling or separation of the coating is noted even under these extreme conditions, thus demonsrating the excellent ductility and adherence of the coating produced with the bath.

EXAMPLE FOUR The bath and procedure described in Example Two are substantially repeated, the sole modification being the substitution of an equi-molar quantity of cadmium chloride for the cadmium oxide employed in accordance therewith. The results achieved are of a comparable quality in all respects.

Additional baths are prepared having comparable compositions to that of Example Two, but utilizing amounts of nitrilotriacetic acid and ethylenediaminetetraacetic acid which, in one instance, provide mole ratios thereof to cadmium of about 1321.0 and 0.17:1.0 respectively, and, in a second instance, provide mole ratios thereof to cadmium of about 3.0: 1.0 and 0.5: 1.0 respectively. In the first case an acceptable deposit is obtained at a high current density, but the deposits are undesirably dull and rough in the middle and low ranges. In the second instance no plating at all occurs in the medium and low ranges, and, although a deposit is formed in the high current density area, it is very poor and evidences burning and pitting so as to be entirely unacceptable.

Thus, it can be seen that the present invention provides a novel and highly effective bath for producing bright, adherent, ductile cadmium deposits on various metallic substrates, is operable with high current efiiciency over a wide range of current density and affords good throwing power and operability at low voltages. The bath is effective for plating cadmium on cast and malleable irons and produces a relatively low level of hydrogen embrittlement in embrittlernent-susceptible ferrous alloys. The present invention also provides a novel and effective method for depositing highly desirable cadmium coatings upon metallic substrates which is relatively economical and trouble-free over extended periods of operation.

Having thus described the invention, We claim:

1. An aqueous electroplating bath for depositing cadmium, comprising about 7.5 to 60.0 grams per liter of cadmium ion, about 16.0 to 135.0 grams per liter of chloride ion, about 0.16 to 1.2 gram moles per liter of an aminopolyacetic acid-type of chelating agent, and at least about 1.0 gram per liter of a nicotinic acid, said bath having a pH of about 5.0 to 7.5.

2. The bath of claim 1 wherein said chelating agent is selected from the group consisting of nitrilotriacetic acid, ethylenediaminetetraacetic acid, trisodium-N-hydroxyethyl ethylenediaminetriacetate, and mixtures thereof, and wherein said nicotinic acid is selected from the group consisting of nicotinic acid, isonicotinic acid, derivatives and mixtures thereof.

3. The bath of claim 2 wherein said chelating agent is nitrilotriacetic acid and said nicotinic acid is isonicotinic acid.

4. The bath of claim 2 wherein said chelating agent is a mixture of a major amount of nitrilotriacetic acid and a minor amount of ethylenediaminetetraacetic acid, and wherein said nicotinic acid is a mixture of a major amount of nicotinic acid and a minor amount of isonicotinic acid.

5. The bath of claim 1 additionally containing a small effective amount of an anionic wetting agent, about 0.5

to 4.0 gram moles per liter of ammonium ion, and about 6.0 to 80.0 grams per liter of an acid selected from the group consisting of citric, boric, tartaric and malic.

6. The bath of claim 1 wherein said chloride ion is at least partly furnished by ammonium chloride.

7. An aqueous electroplating bath for depositing cadmium, comprising about 7.5 to 60.0 grams per liter of cadmium ion, about 16.0 to 135.0 grams per liter of chloride ion, at least about 1.0 gram per liter of a nicotinic acid, and quantities of nitrilotriacetic acid and of a second aminopolyacetic acid-type of chelating agent sufiicient to provide respective weight ratios to cadmium ion of about 1.6 to 2.8:1.0 and 0.12 to 1.0210.

8. The bath of claim 7 wherein said nicotinic acid is present in an amount of about 2.0 to 20.0 grams per liter and is provided by a mixture of nicotinic acid and isonicotinic acid in a weight ratio of about 2.0 to 4.0110, respectively.

9. The bath of claim 8 additionally containing a small, effective amount of an anionic wetting agent and about 6.0 to 80.0 grams per liter of citric acid, and wherein said chloride ion is at least partly furnished by ammonium chloride.

'10. The bath of claim 9 comprising about 15.0 to 40.0 grams per liter of cadmium ion, about 50 to 160 grams per liter of ammonium chloride, about 4.0 to 12.0- grams per liter of said mixture of nicotinic acids and about 15.0 to 60.0 grams per liter of citric acid, wherein said second chelating agent is ethylenediaminetetraacetic acid and wherein the weight ratios to cadmium ion of nitrilotriacetic acid and ethylenediaminetetraacetic acid are 2.0 to 2.6-: 1.0 and 0.250.5:1.0, respectively.

11. The bath of claim 10 wherein said ammonium chloride is present in an amount of about 75.0 to 100.0 grams per liter, said ratios are 2.2 to 24:10 and 0.3 to 0.4: 1.0 respectively and said pH is about 6.0 to 6.8.

12. In a method of electroplating cadmium, the steps comprising:

(a) preparing an aqueous bath having a pH of about 5.0 to 7.5 and comprising about 7.5 to 60.0 grams per liter of cadmium ion, about 16.0 to 135.0 grams per liter of chloride ion, at least about 1.0 gram per liter of a nicotinic acid, and quantities of nitrilotriacetic acid and of a second aminopolyacetic acid- 8 type of chelating agent sufiicient to provide respective weight ratios to cadmium ion of about 1.6 to 2.8:1.0 and 0.12 to 1.0110;

(b) maintaining said bath at a temperature of about to |120 Fahrenheit;

(c) immersing a workpiece having a metallic surface and a cadmium anode in said bath; and

(d) applying a voltage across said anode and workpiece to deposit cadmium on said metallic surface.

13. The method of claim 12, wherein said nicotinic acid is present in an amount of about 2.0 to 20.0 grams per liter and is provided by a mixture of nicotinic acid and isonicotinic acid in a weight ratio of about 2.0 to 4.0:1.0, respectively, wherein said bath additionally contains a small, effective amount of anionic Wetting agent and about 6.0 to 80.0 grams per liter of citric acid; and Wherein said chloride ion is at least partly furnished by ammonium chloride.

14. The method of claim 13 wherein said bath comprises about 15.0 to 40.0 grams per liter of cadmium ion, about 75.0 to 100.0 grams per liter of ammonium chloride, about 4.0 to 12.0 grams per liter of said mixture of nicotinic acids and about 15.0 to 60.0 grams per liter of citric acid, wherein said second chelating agent is ethylenediaminetetraacetic acid, and wherein the Weight ratios to cadmium ion of nitrilotriacetic acid and ethylenediaminetetraacetic acid are 2.02.6:1.0 and 0.25-0.5:1.0, respectively.

15. The process of claim 12 wherein said temperature is maintained between about and Fahrenheit, wherein the pH of said bath is about 6.0 to 6.8, and wherein said applied voltage produces a current density at said workpiece of about 25.0 to 40.0 amperes per square foot.

References Cited FOREIGN PATENTS 565,427 10/1958 Canada 204-43 OTHER REFERENCES Larissa Domnikov, Metal Finishing, pp. 62-64, November 1969.

GERALD L. KAPLAN, Primary Examiner 

